The present invention relates to an image display apparatus which allows a user to observe an image formed by an image-forming element through an ocular unit (eyepiece).
Image display apparatuses such as a head-mounted display and an electronic viewfinder of a digital camera typically include a two-dimensional display element such as a transmissive liquid crystal element, a reflective liquid crystal element, and an organic EL element. In many image display apparatuses, the two-dimensional display element is used in combination with an ocular unit to allow observation of a displayed image as a virtual image. In addition, retina projection image display apparatuses have been proposed in which an image is formed directly on the retina of an eye of an observer by using a scanning element for two-dimensionally scanning light from a light source.
In such an image display apparatus, especially having a wide field of view for observation, when an observer widely moves his eyeballs in attempting to observe the end portion of the field of view, his pupils are also moved greatly in association with the eyeball movement. If the diameter of the exit pupil of the image display apparatus is small, a light flux emerging therefrom proceeds out of the pupils to prevent the observation of the image. To avoid this, the ocular unit is typically designed to provide the exit pupil having a large diameter. In a scanning image display apparatus which tends to have a small exit pupil in principle, an enlarging means such as a lens array and a diffusing plate is used to increase the divergence angle of an emerging light flux (see U.S. Pat. No. 5,701,132).
When the exit pupil having a large diameter is provided, the diameter is set to a value equal to or larger than the diameter of the pupils of an observer. Thus, if the pupils are moved with the eyeball movement, an emerging light flux always enters the pupils to allow an observer to observe an image without vignetting. However, as the diameter of the exit pupil is larger, the proportion of light entering the pupils of an observer is smaller, resulting in a darker image to be observed.
In an image display apparatus as described above including a diffusing plate disposed on an intermediate image-forming plane, when a user observes an enlarged image through an ocular unit, the pattern of the diffusing plate is observed over the displayed image to reduce the quality of the observed image.
In an image display apparatus having the exit pupil with a large diameter, if an observer has a problem in his eyesight adjusting function such as short sight, long sight, astigmatism, and presbyopia, the observer must wear glasses or contact lenses in observing an image with an image display apparatus. This makes the observer feel intrusive.
To address this, an image display apparatus has been proposed in which the diameter of an exit pupil is set to a small value and an observer can see an image with the Maxwellian view. In the Maxwellian view, light from a light source is once converged by a lens to the pupils of an observer and then caused to enter the eyeballs to allow the observer to see the light. When an optical system for enabling image observation with the Maxwellian view is used, the observer can see an image with high resolution regardless of the eyesight of his eyes. This is because a thin light flux enters the pupils and thus the image observation is not affected by the eyesight condition such as short sight, long sight, astigmatism, and presbyopia. In addition, the light emerging from a light source can be effectively caused to enter the pupils to allow observation of a very bright image.
Even when the Maxwellian view is used, however, the emerging light flux does not enter the pupils and the observation of an image is prevented if an eyeball movement occurs or if the relative positions of the observer and the image display apparatus are changed.
To solve the problems, Japanese Patent Laid-Open No. 9(1997)-171147 has proposed an image display apparatus in which a variable-angle prism is placed at the focal point of an ocular unit and the angle of the variable-angle prism is changed to move the exit pupil.
In the image display apparatus disclosed in Japanese Patent Laid-Open No. 9(1997)-171147, however, when an image with a wide viewing angle is assumed, the pupils of an observer who attempts to see the end portion of the image are moved widely. If the angle of the variable-angle prism is greatly changed in association with that large moving amount, significant distortion occurs in the image to make the observer feel strange. In addition, a large variable amount of the angle of the variable-angle prism increases the size of the variable-angle prism to cause an increase in the overall size of the system.